Alicq

Alicq is pure Tcl/Tk implementation of ICQ client with flexible modularized architecture, support for ICQ2000/Oscar protocol, and ability to pick up Licq configuration files and user database.

Why would you use it? It is small, about 4500 lines as of release 0.8.5. It is modular and extendable in best tradition of the Tcl scripting language. New modules are easy to write, and dozen lines Tcl module can do a lot. It is cross-platform: works on Unix, Windows, Macintosh, and on any other system supported by Tcl/Tk.